1. Filed of the Invention
This invention relates to a heavy duty pneumatic tire capable of reducing noise during running over a long period of time.
2. Description of Related Art
Recently, noise is recognized as a pollution problem from operation of an automobile. The noise generated during the running of the tire is a sound mainly based on air discharged from grooves formed in a tread between a tire tread and a road surface. The tire tread is comprised by endlessly and circumferentially arranging tread-constituting elements such as plural circumferential grooves, land portions defined by these grooves such as ribs, blocks and the like, and sipes formed in these land portions and so on.
Since air existing in the groove between the ribs or the blocks and in the sipe vibrates at a ground contact region of the tread with the rotation of the tire during the running of the tire, a pitch of sound or frequency is determined by the ground contacting number of the tread-constituting element per 1 second, which comes to person's ears as a certain sound level. Therefore, it is usually attempted to disperse a particular frequency into unobtrusive sound by applying pitch variation technique based on frequency modulation theory using different element lengths or different pitches to the tread-constituting element.
On the other hand, high speed running is increasing with the remarkable servicing of express way networks. In this case, wide-width and straight circumferential grooves are mainly used as a groove defining the land portion of the tread from a viewpoint of consideration on wet-skid resistance. However, such a circumferential groove creates columnar resonance therein, which results in a new noise problem. That is, the width of the circumferential groove in the ground contact region of the tread is narrowed at a stepping-in side and returned to an original width at a kicking-out side during the running of the tire under loading. Hence air existing in the circumferential groove at the ground contact region becomes dense at the stepping-in side and coarse at the kicking-out side to always generate an acoustic resonance sound as a stationary wave in the ground contact region, which brings about the occurrence of noise.
In order to prevent the occurrence of noise through columnar resonance, it has attempted in a heavy duty pneumatic tire for truck and bus that flexible groove fences, each formed by extending a part of a tread rubber from a groove wall or a groove bottom in a direction of blocking an air flow, are arranged in the circumferential groove at given intervals in the circumferential direction of the tread. As shown in FIGS. 6a and 6b, the conventional groove fences are double door type rubber fences 3 formed by extending a part of a land portion from opposed walls 2 of a circumferential groove 1 so as to leave a space of about 1 mm in a widthwise center of the circumferential groove. In general, the conventional groove fences block the columnar resonance by closing the door fences during the running under loading.
The conventional groove fences shown in FIGS. 6a and 6b have an effect of reducing the above columnar resonance, but cause another problem in that the door fences 3 contact with each other when the width of the circumferential groove 1 is reduced in the ground contact region under loading. Hence crack 4 is created in the vicinity of a boundary between the door fence 3 and the groove wall 2 as shown in FIG. 9 and finally the door fence 3 comes off of the groove wall 2 with the growth of the crack.